1. Field of the Invention
This invention relates to a process for preparing stable ultramarine, metal powder and other inorganic pigment compositions the particles of which are coated with fine amorphous silica and which has good pigment characteristics, i.e., high chemical resistance, hydrogen sulfide resistance, light resistance, weatherability, heat resistance and storage stability.
2. Description of the Prior Art
Pigments are widely used as colorants for paints, printing ink and various articles such as plastics, rubber, construction materials, cosmetics and paper. They are also used as an agent for providing protective coatings because of their advantageous physical and chemical properties such as hiding power, covering power, reflecting properties, insulating effect and rust preventing power. Ultramarine (Color Index No. 77007) is a pigment with a blue to purplish pink hue and it consists mainly of silicate, alumina and sodium oxide plus sulfur. Because of its very low toxicity to man and animals, the pigment has recently gained increasing attention for use as a colorant not only for construction materials but also for use in synthetic resin paints, printing inks, rubbers, artists' paints, cloths, paper, synthetic detergents, textiles and cosmetics. Metal powder pigments in a flake form and having characteristic metallic colors are primarily used for ornamental coatings or corrosion inhibition of molded plastics, printing inks, paints, cosmetics, wall papers, fans, toys, pictures. However, these pigments have undesired properties of their own, for example, there is the chalking of titanium oxide, the heat- or light-induced discoloration of chrome yellow and yellow oxide and the decomposition of inorganic pigments (e.g., cadmium ultramarine pigments) with acid. Further, metal powder pigments discolor upon exposure to heat, light and hydrogen sulfide and decompose when contacted with acid. For these reasons, the type and amount of pigments used are essentially limited by the manufacturing process and the use of the articles to which they are applied. Conversely, the type of pigments limits the articles to which they can be applied. For example, chlorine gas generated during the manufacture of a sheet of polyvinyl chloride containing ultramarine decomposes the pigment and results in discoloration of the sheet and the generation of hydrogen sulfide. Immersing the ultramarine powder in hydrochloric acid causes rapid decomposition of the pigment accompanied by the generation of hydrogen sulfide.
With the recent improvement in the technology of forming and processing olefinic plastics as well as the use of manufacturing temperatures higher than 250.degree. C., the demand for highly heat-resistant metal powder pigments has increased. Under these circumstances, various industrial sectors have expressed a strong need for an improvement in pigment characteristics such as light resistance, weather-ability, chemical resistance, hydrogen sulfide resistance, heat resistance, and storage stability. Many processes for improving the characteristics of pigments have been proposed and implemented on a commercial scale, but the fact is they have their own advantages and disadvantages. Examples of the methods for improving metal powder pigments are: (1) heat treatment with boric acid and borax or covering the pigment with organic matter as described in Japanese Pat. Nos. 482178, 496788 and 570291, (2) covering the pigment with a transparent metal oxide film containing beryllium and chromium as described in Japanese Pat. No. 509549, and (3) a surface treatment with chromate as described in Japanese Patents 509812 and 924417. However, none of these techniques provide pigments adequately resistant to heat, corrosion and chemicals. Other known improvements use an aqueous solution of an alkali silicate. One of them comprises adding first an aqueous solution of silicate and then boric acid and an aliphatic alcohol during the grinding of the metal powder as described in Japanese Patent Publication No. 40696/72. The method adds a very thick alkali silicate in the course of grinding a pigment and then forms a silica gel, but the gel formed is heterogeneous and coarse and may contain unreacted alkali silicate. As a result, the silica gel coating comes off the pigment easily and has little covering power. Other techniques combine a chromate treatment simultaneously with the coating of a silica as described in Japanese Patent Publication No. 8019/74; and reacting an aqueous solution of silicate with an aqueous solution of an acidic aluminum salt by adding the former to the latter or vice versa to form the silica precipitate, followed by heating to a temperature higher than 200.degree. C. as described in Japanese Patent Publication No. 46775/76. The silica formed in these methods has only weak power to cover the metal powder pigment. This may be the reason the two methods require a chromate treatment and a heat treatment, respectively, to provide a pigment with the desired resistance. The silica formed in these methods is a precipitated silica gel which consists of large particles, is heterogeneous, easily becomes porous, and is therefore difficult to deposit on the pigment particles. A continuous coating of silica is not formed, and the coating easily separates from the pigment particles. As a result, none of the methods described above is completely satisfactory. They do not adequately improve the various resistances required of metal powder pigments.
A method is disclosed in Japanese Patent Publication No. 16531/74 for improving the resistance of inorganic pigments by covering the pigment particles with a metal oxide by means of a hydrothermal treatment with heat and pressure, but the method does not fully achieve the properties desired probably because the particles of the metal oxide grow in size during the hydrothermal treatment. A method described in Japanese Patent Publication No. 9555/71 (corresponding to U.S. Pat. application Nos. 493,295 and 555,954) covers particles of a lead chromate pigment with a fine amorphous silica produced from the reaction of a dilute aqueous solution of sodium silicate and dilute sulfuric acid. The resulting silica coat makes the pigment very resistant in many aspects, but because of the strong acidity of sulfuric acid, the method is not applicable to ultramarine which is sensitive to the acid (the crystal structure of the pigment would be attacked resulting in destruction of the pigment per se). An attempt to preserve the crystal structure of ultramarine from the acid attack by using a very dilute aqueous sulfuric acid is practically impossible because it requires the preparation of a huge volume of sulfuric acid solution which is not economical in commercial operations.
As a result of various studies on a method of coating particles of any inorganic pigment with a protective coat which is free from the defects described above, it has been found that certain compounds, which will not damage the particles of pigment, react with an aqueous solution of alkali silicate and form an extremely fine and reactive silica. The pigment which is coated with such silica is remarkably resistant.